Because Oracle currently tells as it! (and it may be, most likely, an historical reason.)
Anyway, any word that is in this view should not be used because at any time Oracle can change a value from N to Y.

Why this does not answer the question?
Because if there are non-ANSI keywords that are reserved words then the fact that TIMESTAMP is not an ANSI keyword is not a reason for not being reserved. It is just a matter of logic.

Can I explain why TIMESTAMP is not a keyword when DATE is one?
No I can't this is a question to ask to Oracle developers. Maybe they had just forgotten to make it reserved (I doubt about that) or maybe they do not need it to make it CURRENTLY as a reserved word but note that the simple fact that it is in the view proves that one day or another it might be one. And maybe DATE is a reserved word because at the time SQL syntax analyzer (and PL/SQL after it), far before SQL ANSI standard existed, was not smart enough to see the difference between DATE as an identifier and DATE as a datatype and so it is kept from version to version as a reserved word as any basic datatype that existed or have been introduced from V1 to V8i.